I think the advise was generally good so don't panic folks. Most sick fish don't make it unfortunately.
The salt was not a completely bad move and may not have contributed negatively to the issue. Sadly it is all trial and error trying to treat fish diseases. The only part I'd mention on salt again, that was covered by LBdave, is to increase the dose slowly, not all at once.
The salt is good against bacterial outbreaks and also many parasitic but not all fish can take the dose you need to do in order to kill those. Pop eye is mostly bacterial though, or fluid build up for one or another reason, perhaps the injury was too bad and the salt had nothing to do with it?
Alternatively pop-eye is often due to poor water quality if injury is not involved.
If bacterial, and living in a country where antibiotics are available, that'd be my choice, something that covers mostly gram negative but also some gram positive bacs, so broad spectrum.